1. Field of the Invention
The present invention relates to a method and apparatus for signal detection in a Radio Frequency IDentification (RFID) system, and more particularly to a method and apparatus for detecting signals according to frequency bands used in an RFID system.
2. Description of the Related Art
In general, radio frequency systems are used in various fields, such as voice and data communication, which are types of bidirectional communication services, broadcast communication, which is a type of unidirectional communication service, etc. Also, with the development of radio frequency communication technology, the radio frequency systems are evolving toward providing various conveniences to users. The evolution of these radio frequency systems substantially contributes to implementing ubiquitous systems. One radio frequency system contributing to the implementation of the ubiquitous systems is an RFID system. This RFID system is a system that can be used in various applications, such as inventory management, automatic inspection, warehousing/delivery management, product traceability for preventing theft of freights, etc.
FIG. 1 illustrates a conceptual view for explaining the operation of the RFID system.
The RFID system consists of a product 10 to which an RFID tag (hereinafter, “tag”) 11 is attached, an RFID reader (hereinafter, “reader”) 20 with an antenna 21, and a host computer 30 for gathering information of the reader 20. The RFID system is largely divided into an active type in which the tag 11 has its own power supply, and a passive type in which the tag 11 doesn't have its own power supply, but is activated by the electromagnetic field of the external reader 20. The active type RFID system does not require the reader to select a channel, and needs no construction for transmitting an electromagnetic field to the tag, so a description will be given below only of the passive type RFID system.
Also, the passive type RFID system is divided into the American type employing a Frequency Hopping Spread Spectrum (FHSS) scheme, and the European type employing a Listen Before Talk (LBT) scheme, according to the scheme in which the reader selects a channel. The FSSH scheme is a scheme in which the reader 20 transmits a Continuous Wave (CW) signal while randomly shifting a specific channel, and waits for a response thereto from the tag 11. In contrast, the LBT scheme is a scheme in which, in order to select an available channel, the reader 20 checks whether a currently selected channel is occupied by another reader, through an energy measurement method before transmitting a CW signal.
The energy measurement method refers to a method in which whether there is the energy (power) of a CW signal transmitted by another reader is checked for any channel, and the channel is considered an idle channel when power caused by a CW signal from another reader is not detected. The aforementioned two schemes of the passive type RFID system are common in that a CW signal is generated in the reader 20, and a signal fed back from the tag 11 is detected. However, they have a difference in that the FHSS scheme uses a channel randomly selected from among selectable channels, and the LBT scheme uses a channel selected by checking an idle channel from among selectable channels.
If a CW signal is transmitted over a channel selected using the FHSS or LBT scheme, the tag 11 transmits a tag signal to the reader 20 by using the CW signal, and thereby the reader 20 acquires the tag signal, the reader 20 transfers the tag signal to the host computer 30 by wire 25 or radio 36. Here, the reader 20 may employ various schemes for transmitting data to the host computer 30 by radio 26.
Hereinafter, the structure and operation of a general reader 20 will be described in detail. FIG. 2 illustrates the internal structure of a Radio Frequency (RF) transmitter/receiver unit of a general reader in an RFID system.
The structure of the reader 20 illustrated in FIG. 2 includes only a structure for signal transmission/reception with the tag 11. That is, it should be noted that parts connected with the host computer 30 are not illustrated in the drawing. Also, the reader 20 of FIG. 2 can support the FHSS scheme, but cannot support the LBT scheme. The reason for this will be described in detail below in conjunction with FIG. 3. Reference will now be made in detail to the structure of FIG. 2 and a procedure of performing the FHSS scheme in the structure of FIG. 2.
The reader 20 of FIG. 2 includes a receiver unit 210, a transmitter unit 230, a Phase Locked Loop (PLL) 221, a directional coupler 201, a power amplifier 202, and a controller unit 240 for controlling the PLL 221.
First, in order to transmit a signal to the RFID tag 11 by the transmitter 230, data to be transmitted is input from I and Q channels to the transmitter 230. That is, level shifters 235 and 236 of the transmitter unit 230 receive I channel input data and Q channel input data, shift them to corresponding levels, and then inputs the level-shifted data into low-pass filters 233 and 234 corresponding to the respective data. Each of the low-pass filters 233 and 234 then filters the input signal according to a predetermined filtering band, and outputs the filtered signal. The filtered signals are input into mixers 231 and 232, respectively. The mixers 231 and 232 receive signals of the I and O channels from the PLL 221, up-convert the filtered signals to a high frequency band, and then output the up-converted signals, respectively.
The signals output from the transmitter unit 230 in this way are input into the power amplifier 202. The power amplifier 202 amplifies the power of the input signal to a transmission power level, and outputs the power-amplified signal, which in turn is input into the directional coupler 201. The directional coupler 201 splits a transmission CW signal according to paths, and outputs the split signals to an antenna 21 to thereby transmit them to the tag 11. Also, a signal received by the antenna 21 is split according to reception paths in the directional coupler 201, and the split signals are input into the receiver unit 210. Reference will now be made to the structure and operation of the receiver unit 210.
A received signal output from the directional coupler 201 is input into mixers 211, 212. The mixers 211 and 212 receives signals for phase locking according to the I and Q channels, output from the PLL 221, and down-convert the input signals to a low frequency band, and then output the down-converted signals, respectively. The down-converted signals are input into Direct Current (DC) removers 213 and 214 according to the respective channels.
The DC removers 213 and 214 remove DC components from the input signals, and then output them. The signals output from the DC removers 213 and 214 are input into low-pass filters/gain controllers 215 and 216 of the I and Q channels. The low-pass filters/gain controllers 215 and 216 filter the input signals corresponding to the respective channels, adjust the gains of the received signals, and then output the gain-adjusted signals. The controller unit 240 controls the PLL 221 to output a desired frequency according to the frequency band of the corresponding channel, and the PLL 221 performs phase locking according to the base frequency of the corresponding channel, generated by the controller unit 240.
In short, the procedure of performing the operation according to FHSS scheme in the aforementioned reader structure is as follows: The controller unit 240 controls the PLL 221 to output a frequency of a desired Frequency band, and subsequently controls the transmitter unit 230 to output a CW signal. Then, the tag 11 is supplied with power from the transmitted CW signal, and transmits a tag signal to the reader. The so-transmitted tag signal is input from the directional coupler 201 to the receiver unit 210 through the antenna 21. The receiver unit 210 processes the received signal by splitting the received signal into signals according to channels and outputting the split signals.
As described in FIG. 2, the reader transmits the CW signal, output from the transmitter unit, through the antenna. However, since it is very difficult to implement impedance matching between the transmitter unit and the antenna, the receiver unit may receive a part of the CW signal output from the transmitter unit. That is, although the CW signal output form the transmitter unit is transmitted through the antenna, there is a reflected signal of the CW signal due to imperfect impedance matching, and thus the receiver unit receives the reflected signal.
Further, since the tag signal uses the electromagnetic filed of the CW signal, the intensity of the signal's power is smaller than that the intensity of the power of the CW signal. Thus, there often occurs a phenomenon that the power of the reflected signal is larger than that of the tag signal. Consequently, in order to improve the reception quality of the tag signal, the DC removers 213 and 214 are configured in such a manner as to remove the reflected signal. However, the reflected signal is not completely removed by the DC removers 213 and 214, but partially passes through them, as a result of which the reflected signal may become noise to the tag signal. Using a low-noise amplifier in the receiver unit may enhance the reception sensitivity of the received signal including the tag signal and the noise signal, but the linearity level of the received signal at the input of the receiver unit is lowered because the noise signal is amplified. It is known that the reader must have a wide range of linearity level in order to be able to accurately detect a received signal. Thus, since using the low-noise amplifier makes the range of a linearity level narrow, as mentioned above, the low-noise amplifier is not used in the conventional reader.
Next, problems that the reader has with performing the LBT scheme will be discussed. As mentioned above, the LBT scheme is a scheme in which whether a channel is occupied by another reader it is determines so as to search for a channel to be used by a corresponding reader. However, the conventional reader has a problem in that the conventional reader cannot detect a CW signal transmitted by another reader that occupies a corresponding channel. This will be described with reference to FIG. 3.
FIG. 3 illustrates the frequency spectrum of a received signal in the conventional reader. The frequency spectrum illustrated in FIG. 3 accounts for why the conventional reader cannot support the LBT scheme. In FIG. 3, reference numeral 301 designates a CW signal transmitted by another reader and input into the receiver unit of a corresponding reader, and reference numeral 303 designates a tag signal that is transmitted by the tag 11 that has received the CW signal designated by reference numeral 301. There is a certain frequency difference between the CW signal and the tag signal so as to distinguish between transmission and reception frequencies of the reader. Reference numeral 305 designates frequency responses of the DC removers 213 and 214 of the reader, which show the characteristic of a band-pass filter.
As seen from FIG. 3, the CW signal is mostly removed by the DC removers 213 and 214 because the signal is out of the passband of the DC removers 213 and 214. As a result, the corresponding reader cannot adequately detect the power of the CW signal, and thus cannot know if a corresponding channel is occupied by another reader. Therefore, the LBT scheme cannot be supported by the structure of the conventional receiver unit 210.
Further, in view of the reception sensitivity of a signal received by the reader, the LBT scheme requires a reception sensitivity of about −96 dBm according to the European standard, but the present receiver unit 210 is difficult to support such high sensitivity corresponding to the European standard because the receiver unit 210 does not use a low-noise amplifier for the aforementioned reason. In the end, the reader with the structure illustrated in FIG. 2 has a problem in that the reader can support only the FHSS scheme, and cannot support the LBT scheme.
Further, in the structure of the receiver unit 210 of the conventional reader, a received signal passes through the directional coupler 201 via the antenna 21. However, it is known that the directional coupler 201 reduces the input linearity range of the receiver unit 210 by attenuating a signal output to the receiver unit 210. That is, since the directional coupler 201 attenuates a signal to be input into the receiver unit 210, the reception sensitivity and input linearity of the receiver unit 210 are lowered, which makes it difficult to detect the received signal. Thus, in order to solve the problem of lowering in the input linearity level, the conventional reader uses mixers with a wide range of input linearity, which results in a high cost.
Therefore, the conventional reader constructed as described above has the following problems: firstly, the conventional reader cannot support the LBT scheme because a CW signal from another reader is removed by the mixers 213 and 214. Secondly, the conventional reader does not use a low-noise amplifier so as to ensure a wide range of input linearity, and thus cannot obtain reception sensitivity enough to support the LBT scheme according to the European standard. Thirdly, since a receive signal that has passed through the antenna 21 is input into the receiver unit 210 via the directional coupler 201, the reception sensitivity of the received signal is lowered.